Process for holding an object

ABSTRACT

Disclosed are an object holding process and and an apparatus therefor in which a process strain of the object can be accurately compensated for. When the exposure operation is started, the bottom surface of the object is held by a holding unit. If the object has a strain, the process strain is calculated. Here, if the process strain is larger than a tolerance, the temperature of the object is set to an object setting temperature in a position other than an exposure position in order to contract or expand the object by a predetermined amount for a magnification correction. The bottom surface of the object is then held by the holding unit again, and the exposure of the object is performed in the exposure position.

This application is a continuation-in-part of application No. 08/200,001filed Feb. 22, 1994, now abandoned, which is a division of applicationapplication No. 08/095,484, filed Jul. 23, 1993, now U.S. Pat. No.5,329,126 issued Jul. 12, 1994, which is a continuation of applicationNo. 07/854,402, filed Mar. 19, 1992, now U.S. Pat. No. 5,223,039.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus and a process for holdingan object such as a substrate and a wafer, and in particular, to anapparatus and a process for holding a substrate suitable to be used in aproximity exposure apparatus or aligner.

2. Related Background Art

In recent years, there has been a growing need to improve refinedworking techniques in semiconductor device or IC fabrication apparatusesas the capacity of semiconductor memories is enhanced. However, in orderto improve the refined working techniques of the IC fabricationapparatuses, an enhancement of alignment accuracy between a wafer and amask as well as a high resolution of a light source is required.

In order to achieve the high resolution of a light source, it ispossible to use a synchrotron radiation light as a light source in placeof a far ultra violet radiation and the like conventionally used inexposure apparatuses (see, for example, Japanese Laid-open (Kokai) No.2-100311). On the other hand, in order to attain the enhancement of thealignment accuracy, it is necessary to compensate for process strainsresulting from ion milling, etching and the like as well as to reduce apositional detection error by the improvement of accuracy in analignment system for performing the alignment between a wafer and amask.

In the light of the above facts, following systems or processes havebeen presented as a process for vacuum-holding a wafer in which therefined working techniques of the IC fabrication apparatus are improved.

(1) Process strains are compensated for by independently controllingtemperatures of a wafer and a mask to expand or contract them. Thus, theprocess strains are compensated for by a magnification correction (see,for example, Japanese Laid-open No. 53-98782).

(2) Air is caused to flow between a wafer and a mask to generate an aircurtain. Hence, the temperature of at least one of the wafer and themask is controlled to expand or contract them, and a magnificationcorrection is conducted. As a result, process strains are compensatedfor (see, for example, Japanese Laid-open No. 55-123131).

(3) After a wafer is vacuum-held by a wafer chuck, the vacuum-holding isonce released. Temperatures of the wafer and the wafer chuck are thenmade equal to each other (see, for example, Japanese Publication(Kokoku) Nos. 1-14703 and 1-52898).

Following problems, however, occur in a case where each of theabove-discussed wafer vacuum-holding processes is applied to an X-rayexposure apparatus using the above-mentioned synchrotron radiation lightas a light source which is one of proximity type exposure apparatuses.

(1) In the process disclosed in the Japanese Laid-open No. 53-98782,when the wafer and the mask are arranged with a very narrow proximitygap (normally 10 to 100 μm) therebetween as in the above-discussed X-rayexposure apparatus, the temperature of the mask will also be changed ifthat of the wafer is varied. Therefore, it is difficult to independentlychange their temperatures.

(2) In the process disclosed in the Japanese Laid-open No. 55-123131, aproximity gap is extremely small. As a result, it is difficult to createthe air curtain between the wafer and the mask. Further, there is apossibility that the mask will be bent.

(3) In the processes disclosed in the Japanese Publication Nos. 1-14703and 1-52898, a thermal strain of the wafer can be removed. However,since the correction of a magnification cannot be performed, it isimpossible to compensate for the process strains.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an apparatus and aprocess for holding an object such as a substrate in which processstrains can accurately be compensated for.

According to one aspect of an object holding process of the presentinvention, in the process for holding an object on an object holdingunit the temperature of which is controlled, there are performd a firstholding step of holding the object on the object holding unit in a firstposition such as an exposure position, a process strain measuring stepof measuring a process strain of the object held on the object holdingunit in the first position, a detecting step of detecting whether or notthe measured process strain falls within a tolerance, a hold releasingstep of releasing the hold of the object held on the object holding unitwhen the measured process strain falls without the tolerance, atemperature setting step of setting the temperature of the releasedobject to a second predetermined temperature in a second position basedon the measured process strain, and a second holding step of holding theobject on the object holding unit. In the first holding step, the objectholding unit is set to a first predetermined temperature such as anexposure atmosphere temperature, in the temperature setting step, thesecond position is different from the first position, and in the secondholding step, the temperature of the object is set to the secondpredetermined temperature while the object holding unit is set to thefirst predetermined temperature.

According to another aspect of an object holding process of the presentinvention, in the process for holding an object on an object holdingunit the temperature of which is controlled, there are performed a firstholding step of holding the object on the object holding unit in a firstposition such as an exposure position, a process strain measuring stepof measuring a process strain of the object held on the object holdingunit in the first position, a detecting step of detecting whether or notthe measured process strain falls within a tolerance, a temperaturesetting step of setting the temperature of the object to a secondpredetermined temperature in a second position based on the measuredprocess strain by the object holding unit when the measured processstrain falls without the tolerance, a hold releasing step of releasingthe hold of the object held on the object holding unit, and a secondholding step of holding the object on the object holding unit. In thefirst holding step, the object holding unit is set to a firstpredetermined temperature such as an exposure atmosphere temperature, inthe temperature setting step, the second position is different from thefirst position and the object is held on the object holding unit, and inthe second holding step, the temperature of the object is set to thesecond predetermined temperature while the object holding unit is set tothe first predetermined temperature.

According to further another aspect of an object holding process of thepresent invention in the process for holding an object on an objectholding unit the temperature of which is controlled, there are performeda process strain estimating step of estimating a common process strainof a group of objects, a detecting step of detecting whether or not theestimated process strain falls within a tolerance, a temperature settingstep of setting the temperature of the objects to a first predeterminedtemperature contained in an object cassette based on the estimatedprocess strain, and a holding step of taking out the object from theobject cassette and holding the object on the object holding unit. Inthe holding step, the temperature of the object is set to the firstpredetermined temperature and the object holding unit is set to a secondpredetermined temperature such as an exposure atmosphere temperature,and the process strain estimating step comprises, for example, a firstholding step of holding one of the objects on the object holding unit ina first position such as an exposure position in which the objectholding unit is set to the second predetermined temperature and thefirst position is different from a position of the object cassette, anda process strain measuring step of measuring a process strain of the oneobject held on the object holding unit in the first position.

According to one aspect of an object holding apparatus of the presentinvention, there are provided an object holding unit for holding anobject, a process strain measuring unit for measuring a process strainof the object held on the object holding means in a first position suchas an exposure position, a detecting unit for detecting whether or notthe measured process strain falls within a tolerance, and a temperaturesetting unit for setting the temperature of the object released from theobject holding unit to a second predetermined temperature in a secondposition other than the first position based on the measured processstrain. The object holding unit is movable between the first positionand another position and the temperature of the object holding iscontrollable, and the object holding unit is set to a firstpredetermined temperature such as an exposure atmosphere temperature inthe first position.

According to another aspect of an object holding apparatus of thepresent invention, there are provided an object holding unit for holdingthe object which is movable between a first position and anotherposition and the temperature of which is controllable, a process strainmeasuring unit for measuring a process strain of the object held on theobject holding unit in the first position wherein the object holdingmeans is set to a first predetermined temperature such as an exposureatmosphere temperature, and a detecting unit for detecting whether ornot the measured process strain falls within a tolerance, and in thisapparatus, the object holding unit sets the temperature of the objectheld by the object holding unit to a second predetermined temperature inthe another position other than the first position based on the measuredprocess strain when the measured process strain falls without thetolerance, and after the hold of the object by the object holding unitis once released and the object holding unit is returned to the firstpredetermined temperature, the object holding unit again holds theobject and returns to the first position.

These advantages and others will be more readily understood inconnection with the following detailed description, claims and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the structure of an X-ray exposureapparatus to which the present invention is applied.

FIG. 2 is a schematic view showing the structure of a hand shown in FIG.1.

FIG. 3 is a schematic view showing the structure of a temperatureadjusting plate shown in FIG. 1.

FIG. 4 is a flow chart explaining the operation of the exposureapparatus shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows an embodiment of an apparatus and a process forvacuum-holding a substrate of the present invention. The structure of anX-ray exposure apparatus illustrated.

The X-ray exposure apparatus is a type that is disclosed, for example,in the Japanese Laid-open No. 2-100311 which uses a synchrotronradiation light as a light source and in which a wafer 1 and a mask 12are exposed as they stand upright.

In the X-ray exposure apparatus, there are arranged a wafer cassette(not shown) disposed at a left side of FIG. 1 and containing a pluralityof wafers or substrates, a hand 4 for taking out the wafer 1 one by onefrom the wafer cassette and vacuum-holding the periphery of the wafer 1to transfer the same as shown in FIG. 2, a wafer chuck 5 for receivingthe wafer 1 from the hand 4 at a delivery position indicated by a solidline in FIG. 1 and vacuum-holding the bottom surface of the wafer 1, aguide 3 disposed between the wafer cassette and the wafer chuck 5 towhich the hand 4 is slidably mounted in directions of arrows shown inFIG. 1, and an alignment unit 11 for conducting the alignment betweenthe mask 12 and the wafer 1 vacuum-held by the wafer chuck 5 moved to anexposure position indicated by dotted lines in FIG. 1. The wafer chuck 5is settled on a stage 6 and the wafer chuck 5 is a substratevacuum-holding unit the temperature of which is controlled. The abovestructure is the same as that of a conventional X-ray exposureapparatus.

In the above structure, an exhaust pipe 7 is provided in the wafer chuck5 to connect a chuck surface of the wafer chuck 5 where the wafer 1 isvacuum-held and a vacuum source (not shown) to each other. The exhaustpipe 7 is branched into a plurality of portions in the wafer chuck 5 andthe branched portions are opened at the chuck surface. Further, there isarranged a water conduit 8 for circulating temperature-adjusted watersupplied from a temperature-adjusted water circulating unit 9 tomaintain the temperature of the wafer chuck 5 at a constant value. Inthe alignment unit 11, laser beams 20 are applied to alignment marksformed on the wafer 1 and the mask 12, respectively, and the positionaldeviation between them is detected using the reflected laser beams (see,for example, Japanese Laid-open No. 2-100311).

This X-ray exposure apparatus differs from the conventional X-rayexposure apparatus in that there are arranged a temperature adjustingplate 10 disposed near the wafer delivery position for adjusting thetemperature of the wafer 1 and a micro-computer (CPU) 14 to which thealignment unit 11 and the temperature adjusting plate 10 areelectrically connected through first and second interfaces 13₁ and 13₂,respectively.

As illustrated in FIG. 3, the temperature adjusting plate 10 consists ofa contact base 21 with which the bottom surface of the wafer 1 is to bebrought into contact at the time of the temperature adjustment, aheating-cooling element 22 embedded in the contact base 21 parallel to acontact surface thereof (a surface where the wafer 1 is to be broughtinto contact) and made of a Peltier element, a thermal resistance bodyor the like, a temperature sensor 23 embedded in the contact base 21near its contact surface and composed of a thermistor or the like, and apower supply 24 for supplying a control current to the heating-coolingelement 22. The power 24 is electrically connected to theheating-cooling element 22, the temperature sensor 23 and the CPU 14,and supplies the control current based on a difference T_(w) -T betweena wafer setting temperature T_(w) fed from the CPU 14 and a temperatureT detected by the temperature sensor 23.

The operation of this embodiment will be explained referring to a flowchart of FIG. 4.

(1) First vacuum-holding step

When an exposure operation is started, the hand 4 takes out the wafer 1from the wafer cassette and carries the wafer 1 to the delivery position(step 40). In the delivery position, the wafer 1 is received by thewafer chuck 5, and its bottom surface is vacuum-held (step 41). At thistime, since the temperature of the wafer chuck 5 is kept constant by thetemperature-adjusted water, the temperature of the wafer 1 is alsomaintained at this temperature.

(2) Process strain measuring step

The stage 6 is moved to the exposure position by a driving unit (notshown), and the wafer 1 vacuum-held by the wafer chuck 5 settled on thestage 6 is moved to the exposure position (step 42). The alignmentbetween the wafer 1 and the mask 12 is then performed by the alignmentunit 11 using the alignment marks formed on the wafer 1 and the mask 12,respectively. Here, when the wafer 1 has strains, the interval betweenthe alignments formed on the wafer 1 will not be coincident with thatbetween the alignment marks formed on mask 12. Thus, the alignmentcannot be achieved. Therefore, in this case, the interval L, between thealignment marks formed on the wafer 1 is measured by the alignment unit11 in order to measure the process strain ΔL. The process strain ΔL=(L₁-L₀)/L₀ is then calculated by the CPU 14 using the measured interval L₁and a regular interval L₀ (step 43).

(3) Detecting step

If the process strain ΔL measured in the process strain measuring stepfalls within a tolerance or not is detected (step 44), and the exposureof the wafer 1 is started if the process strain ΔL is less than thetolerance (step 49). On the other hand, if the process strain ΔL islarger than the tolerance, following vacuum-hold releasing andtemperature setting steps will be conducted.

(4) Vacuum-hold releasing step and temperature setting step

In the CPU 14, the wafer setting temperature T_(w) is calculated in thefollowing manner (step 45). The wafer setting temperature T_(w) isobtained by:

    T.sub.w =T.sub.R +ΔL/α                         (1)

where α is a coefficient of linear expansion of the wafer 1 and T_(R) isan exposure atmosphere temperature. If, for example, the process strainis equal to ΔL=10⁻⁶, the coefficient of linear expansion α=2.4×10⁻⁶(1/°C.) and the exposure atmosphere temperature T_(R) =23.0 (°C.), thewafer setting temperature T_(w) =23.42 (°C.) is obtained from theequation (1). After the wafer setting temperature T_(w) has beenobtained, the control current according to the difference T_(w) -Tbetween the wafer setting temperature T_(w) and the temperature Tdetected by the temperature sensor 23 is output to the heating-coolingelement 22 by the power 24 shown in FIG. 3, and the temperature of thetemperature adjusting plate 10 is thus set (step 46). Further, the stage6 is moved to the delivery position by the driving unit, and the wafer 1vacuum-held by the wafer chuck 5 is thus returned to the deliveryposition (step 47). Thereafter, the vacuum-holding of the wafer 1 by thewafer chuck 5 is released (vacuum-hold releasing step), and the wafer 1is then handed to the hand 4. The wafer 1 is carried onto the contactsurface of the temperature adjusting plate 10 by the hand 4, and thebottom surface of the wafer 1 is brought into contact with the contactsurface of the plate 10 (step 48). When the bottom surface of the wafer1 is brought into contact with the contact surface, the temperature ofthe wafer 1 is set to the wafer setting temperature T_(w) (=23.42° C.).As a result, the wafer 1 is expanded by a predetermined amount, and themagnification correction is completed (temperature setting step).

(2) Second vacuum-holding step

After the temperature of the wafer 1 is set to the wafer settingtemperature T_(w), the wafer 1 is again moved to the delivery positionby the hand 4 and then handed to the wafer chuck 5. The bottom surfaceof the wafer 1 is thus vacuum-held by the water chuck 5 (step 41). Atthis time, since the temperature of the wafer chuck 5 is maintained atthe exposure atmosphere temperature T_(R) (=23.0° C.) by the temperatureadjusted water, the wafer 1 would not contract even if this is liable tocontract for the wafer 1 is vacuum-held by the wafer chuck 5 with astronger force than this liability. Afterwards, the steps 42 to 44 areagain performed. The exposure is started if it is confirmed that theprocess strain ΔL falls within the tolerance range in the step 44 (step49).

In the foregoing, the heating-cooling element 22 consisting of a Peltierelement, a thermal resistance body or the like is used in order toconduct the temperature setting of the temperature adjusting plate 10.However, in place of the heating-cooling element 22, thetemperature-adjusted water may be circulated in the contact base 21 (seeFIG. 3) similar to the wafer chuck 5.

Further, the wafer chuck 5 say also be caused to act as the temperatureadjusting plate 10. In this case, however, the wafer chuck 5 should bemade of a material whose coefficient of linear expansion α_(c) isdifferent from the coefficient of linear expansion α of the wafer 1, andthe wafer setting temperature T_(w) in the temperature setting step isobtained by:

    T.sub.w =T.sub.R +ΔL/(α-α.sub.c)         (2).

The temperature of the wafer 1 is thus brought to the wafer settingtemperature T_(w).

Next, the vacuum-holding of the wafer 1 by the wafer chuck 5 is oncereleased in the vacuum-hold releasing step, and after the temperature ofthe wafer chuck 5 is again set to the exposure atmosphere temperatureT_(R), the wafer 1 is again vacuum-held by the wafer chuck 5 in thesecond vacuum-holding step. The wafer 1 vacuum-held by the wafer chuck 5is then returned to the exposure position. The remaining is the same asthe above-discussed embodiment with the exception that thetemperature-adjusted water circulating unit 9 is electrically connectedto the CPU 14 through the second interface 13₂.

Although not shown in FIG. 1, an orientation flat detecting unit foradjusting the orientation of the wafer 1 may be arranged between thewafer cassette and the wafer chuck 5 as is often the case with this kindof an X-ray exposure apparatus (see, for example, Japanese Laid-open No.2-100311). In such a case, it is also possible to cause the orientationflat detecting unit to act as the temperature adjusting plate 10.Moreover, it is also possible to cause the hand 4 to act as thetemperature adjusting plate 10. In these cases, the orientation flatdetecting unit and the hand 4 are electrically connected to the CPU 14through the second interface 13₂, respectively.

The compensation operation of the process strain may be performed foreach wafer, but in a case where there is little divergence of theprocess strains in the same lot (all wafers 1 contained in the wafercassette), the operation may be performed as follows: The compensationoperation of the process strain is conducted only for a wafer 1initially taken out of the wafer cassette. For the second wafer onwardin this wafer cassette, the temperature setting (see the step 46) isperformed in the wafer cassette, and the wafer 1 is then taken out bythe hand 4 one by one. Here, the temperature of the wafer chuck 5 ismaintained at the exposure atmosphere temperature T_(R). Naturally, thewafer cassette is electrically connected to the CPU 14.

If the wafer 1 is strained anisotropically, the temperature distributionof the temperature adjusting plate 10 may be set in accordance with suchstrain of the wafer 1.

In the above embodiment, the description is made as to the X-rayexposure apparatus using a synchrotron radiation light as a lightsource, but the present invention may also be applied to other exposureapparatuses such as a proximity exposure apparatus using an ultra violetlight as a light source. Further, an object the process strain of whichis to be compensated for is not limited to the wafer, but the object maybe other objects such as a substrate of a liquid crystal display or thelike on which thin film transistors and the like are to be formed.

The following advantages can be obtained by the structure of the presentinvention.

Since the above-discussed first vacuum-holding step, process strainmeasuring step, detecting step, vacuum-hold releasing step, temperaturesetting step and second vacuum-holding step are performed in the processand the apparatus for vacuum-holding a substrate of the presentinvention, the temperature of a mask would not be adversely affectedeven when the substrate and the mask are positioned with a very narrowproximity gap therebetween. Therefore, the temperature of the substratecan be set based on process strains of the substrate, and hence themagnification correction can be accurately achieved by expansion orcontraction of the substrate. As a result, the process strains can beaccurately compensated for.

While the present invention has been described with respect to what ispresently considered to be the preferred embodiment, it is understoodthat the invention is not limited to the disclosed embodiment. Thepresent invention is intended to cover various modifications andequivalent arrangements included within the spirit and scope of theappended claims.

What is claimed is:
 1. A process for holding and exposing an object,comprising the steps of:holding the object on a chuck; measuring astrain of the object held on the chuck; detecting whether the measuredstrain falls within a tolerance range; performing the following stepswhen the measured strain falls out of the tolerance range:releasing theholding of the object; correcting the size of the object; and holdingthe object again on the chuck while maintaining the corrected size; andexposing the object held on the chuck.
 2. A process according to claim1, wherein said object is a wafer, and wherein said exposing stepcomprises the step of forming a pattern on the wafer.
 3. A processaccording to claim 2, wherein said exposing step is performed when amask and the wafer are positioned close to one another.
 4. A processaccording to claim 3, wherein said exposing step comprises the step ofexposing the object to an x-ray beam.
 5. A process according to claim 1,wherein the strain is a process strain.
 6. A process according to claim1, wherein said correcting step comprises the step of changing thetemperature of the object.
 7. A process for holding and exposing anobject, comprising the steps of:holding the object on a chuck; releasingthe object from the chuck; correcting the size of the object by settingthe temperature of the object which is different from the temperature ofthe chuck; holding the object again on the chuck while maintaining thecorrected size regardless of the temperature of the object; and exposingthe object held on the chuck with radiation.
 8. A process according toclaim 7, wherein said object is a wafer.
 9. A process according to claim7, wherein said object is a wafer.
 10. A process for holding andexposing an object, comprising the steps of:holding the object on achuck; releasing the object from the chuck; correcting the size of theobject; holding the object again on the chuck while maintaining thecorrected size regardless of the temperature of the object by a holdingforce which is stronger than the contracting or expanding force of theobject; and exposing the object held on the chuck with radiation.